I rescued Toothless from my work, and his tail appears to be growing back a bit. Just curious though about how it's growing back. I was told it might not at all/much because he lost his rays and those don't tend to grow back.
Here he is NOW:

Here he is the day I bought him:

It's growing in curled. Is there any way I can prevent this? Or is it just going to grow how it pleases?

I don't think there's much else you can do aside from keeping his water pristinely clean and giving him some high quality high protein foods to help him regenerate his tail. I suggest feeding this guy some live and frozen foods while his tail is growing--live blackworms are available at most pet stores--rinse thoroughly before giving them to your betta. Frozen blood worms and brine shrimp are also good, be sure to thaw out (just a small portion) before feeding to your fish. More protein in his diet should contribute to the strength of his new tail growth. Warm temperatures should also stimulate more growth.

Actually now that I look more closely, your fish appears to be a little bloated/fat.. Increase the quality of the food and decrease the quantity. If his tank is small, I suggest upgrading to a larger size to help with the water quality and to help get him some exercise.

Yeah, we'd gotten a new shipment of bettas and this poor guy stuck out like a sore thumb. I took him home ASAP. Not sure what happened to him ):
He's in about 4g (+/- .5g) of a divided 10-gallon. I'll keep an eye out for the blackworms, though I haven't seen them. I work at a fish store and we've got tons of bloodworms. I might have to pick up some frozen ones. I've also got the Marineland Stealth Pro heater in there, keeping the water a steady 81F.
As for the bloating, the silly boy devoured one of my otos algae wafers. I only feed him 3/4 Hikari betta pellets a day (soaked), occasional freeze-dried bloodworms (2) maybe once every two weeks. I fast both my bettas 1x/week.

I dont know if the tail will grow back but if it does it will most likely be wavey and irregular. That's what happened to one of my boys after he got a bad case of finrot, he lost about half his tail and it has fully regrown.

Some people reckon the curling is caused by hard water and can be remedied by a product called 'Trace' which I can't seem to find a link for, but basically it replaces all the trace elements in healthy amounts like zinc, copper etc. To be honest it sounds somewhat questionable but its worth a shot if its important to you that he has the best regrowth possible. I am sure there are many versions of this product on the market.

Good luck with it all, I can't get over how lucky he is to have gotten you!!

Thanks everyone. All I want is for him to be happy and healthy and live as long as he can. I don't care how his tail looks when/if it grows in, I just don't want him to have to struggle
with if it it's going to be functionless weight.

The only time it's a bother is when he charges up to the top for food. He has to wiggle his bum a lot harder than Roscoe does. Other than that, casual swimming looks no different than Roscoe.

I'll have to pick up a test kit for hardness and check it out. If it is hard, I'll do a bit of research on "Trace"

My male vt had a terrible case of fin rot, losing almost all his fins :O. (I'll never leave him in the care of others again!). His dorsal, pec, and ventral fins grew back normally, his tail fin barely grew back at all and seems to be permanently clamped, and his anal fin grew back long and nice, but comes completely to a curl that tucks up just under his tail fin. It actually looks pretty cute when he flares.

Other than the weird fins though, with frequent water changes, good food, and a lot of bettafix, he is happy and healthy and has great colour. I wouldn't be too worried that it will have long term effects on your guy, as it hasn't with mine. He'll adjust to using his other fins more to compensate for the lack of a tail.